Clutches are utilized in a variety of settings. For instance, fan clutches are used in automotive settings to permit selective engagement of a fan to help cool an engine, and viscous fan clutches are commonly used with medium to heavy duty trucks. Generally speaking, these viscous fan clutches operate by introducing a shear fluid to a working chamber to frictionally engage two components, such as a rotor connected to a drive input and a housing connected to a fan, by transmitting rotational energy via the shear fluid. Such a viscous clutch can engage rotation of the fan when shear fluid is present in the working chamber and disengage rotation of the fan when the shear fluid is removed from the working chamber.
Many known viscous clutches are electromagnetically actuated. That is, these viscous clutches include an electromagnetic coil that can generate magnetic flux to control the operation of a valve that, in turn, regulates flow of shear fluid from a fluid supply reservoir to the working chamber. However, there are numerous difficulties in locating the valve and the coil in such a way that the rotational capabilities of the drive are adequately maintained, while also permitting efficient and effective control of fluid flow with the valve.
For example, the reservoir of a viscous clutch is typically attached to the housing of the clutch. Fan blades are connected to the housing. The housing, the reservoir and the fan blades are all generally either stationary or are rotating at a relatively slow speed when the clutch is in an “off” or disengaged condition. A relatively stationary reservoir imparts little kinetic energy to the shear fluid contained inside, which can slow the response time for the clutch to move shear fluid from the reservoir to the working chamber when the valve is opened. But attaching the reservoir to the rotor is problematic, because it is difficult to provide a suitable valve assembly that can rotate with the rotor and yet still be effectively and efficiently controlled by the coil, which generally must be rotationally fixed to enable reliable electrical connections to be made to the coil. Moreover, many flux circuit arrangements for magnetically linking the valve and the coil are undesirable, because the size and power requirements for a coil capable of generating sufficient magnetic flux presents a host of problems. Large coils undesirably add additional weight and cost, and, in addition, can exceed allowable current or voltage requirements for a particular engine, which are typically engine-wide parameters established with regard to the electronic engine controller for the vehicle in which the clutch is installed.